A new rapid assay for phosphatidylserine decarboxylase, which is sensitive in the nanomolar range, is described. Synthesis of radiolabeled phosphatidylserine for use as a substrate is not required, since the assay, unlike previous ones, is based on the detection of CO(2) liberated from unlabeled phosphatidylserine. The assay employs a gas chromatographic procedure for the analysis of methane formed by catalytic conversion of the CO(2) produced as a product of the enzymatic reaction.